1. Field of the Invention
The present invention relates to seals for the floating roofs of storage tanks for petroleum and similar products, and more particularly to an improved secondary seal for use in conjunction with a primary seal.
2. History of the Prior Art
Seals for sealing the space between a floating roof and the inner tank wall of a petroleum storage tank are well known and have been provided in a variety of different configurations. Examples of such seals are provided by U.S. Pat. Nos. 4,308,968, 5,078,293 and 5,103,992, all of which are commonly assigned with the present application. It is common practice in many applications to provide a tank with both a primary seal and a secondary seal. The primary seal is regarded as providing the primary or principal sealing function of the space between the floating roof and the inner tank wall. The secondary seal, which is often installed after the primary seal is in place and the tank has been in service for some time, provides a backup sealing function. Typically, the secondary seal is disposed above the primary seal in order to trap any vapors which may escape through the primary seal. The 1968 patent noted above provides an example of a secondary seal, while the '293 and '992 patents noted above provide examples of primary seals.
The secondary seal described in the '968 patent noted above includes a plurality of overlapping metal plates which have their lower edges mounted on the floating roof and which extend toward the inner tank wall so as to terminate in upper edges disposed adjacent the inner tank wall. A plurality of flexible wiper blades mounted on the upper edges of the overlapping metal plates extend into contact with the inner tank wall. Depending upon whether the tank is of welded or riveted construction, the wiper blades may assume different configurations to accommodate the smoother walls of a welded tank or the irregular wall surfaces of a riveted tank. A vapor barrier fabric disposed beneath the overlapping metal plates has a first edge thereof coupled to the floating roof and an opposite second edge coupled to the upper edges of the overlapping metal plates where the wiper blades are mounted. The vapor barrier fabric seals the space beneath the overlapping metal plates. The overlapping metal plates support the wiper blade arrangement which seals the space beneath the overlapping metal plates at the inner tank wall. U.S. Pat. No. 4,397,399 provides a further example of a seal having a vapor barrier fabric in conjunction with overlapping metal plates which support a single wiper blade.
Secondary seals of the type described in the '968 patent noted above provide effective sealing action, particularly when compared with certain other seal designs including earlier roof seals. However, the elastomeric material typically used for the wipers is subject to relatively rapid deterioration which can greatly shorten the life of the seal that would be otherwise realized with the metal parts thereof. Most elastomers that are resistant to abrasion and other typical tank service conditions are subject to relatively rapid deterioration when exposed to certain gasoline additives and chemicals such as MTBE and benzene. On the other hand, the vapor barrier fabrics are not subject to abrasion, as are the wiper blades, and can be made of laminated fluoropolymers such as Teflon which are resistant to virtually all products stored in floating roof tanks.
Accordingly, it would be desirable to provide an improved secondary seal for floating roof tanks. A more specific object would be the provision of a secondary seal which is free of elastomers or other materials that may be subject to rapid deterioration.